Heat a large skillet over medium-high heat. Cook and stir beef, salt, and pepper in the hot skillet until meat is browned and crumbly, 5 to 7 minutes; drain and discard grease.
Mix ketchup and mustard in a bowl; spread mixture over pizza crust.
Spread mozzarella cheese and Cheddar cheese over ketchup mixture and top with beef and onions.
Transfer pizza to a large baking sheet.
Bake in preheated oven until cheese is melted, about 20 minutes. Top with pickles and allow pizza to cool for 5 minutes. Top with shredded lettuce and tomatoes before slicing and serving.